The main findings of our study are as follows: (i) AF provokes morphological changes and sarcomeric dysfunction in LA myocardium, which is associated with the increased [ROS]i, reduced [NO]i, and decreased phosphorylation of cMyBP-C and TnI; (ii) AF induces LA-to-RA differences in wall thickness, myofibrillar content, EDSL, and sarcomere shortening-relengthening velocities, which could be related to inter-atrial differences in ROS production and contractile protein function but not to changes in [Ca2+]i transients. This evidence concerns the gene MYBPC3 and atrial fibrillation.